Elenco Electronics 202 - Manual (Page 41)

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Project #68
OBJECTIVE: To combine the sounds from the space war and
music integrated circuits.
Build the circuit shown. Turn it on, press the press switch (S2) several
times, and wave your hand over the photoresistor (RP) to hear all the
sound combinations. You can make the sound from the music IC (U1)
louder by replacing the 100
Ω resistor (R1) with the 6V lamp (L2).
Space War Music Combo
Project #67
OBJECTIVE: To convert a changing voltage into a constant
voltage.
Simple Rectifier
Turn on the slide switch (S1) and the LED (D1) lights; it will not be very bright
so turn off the room lights or hold your fingers around it to see it better. Press
the press switch (S2) several times slowly; the LED and lamp (L2) go on and off.
Press the press switch many times quickly - the lamp still goes on and off but
the LED stays on. Next, remove the 470
μF capacitor (C5) from the circuit - the
LED goes on and off now. Why?
Pressing the switch quickly simulates a changing voltage, which turns the LED
on and off. The 470
μF capacitor can store electricity, and it combines with the
NPN transistor (Q2) to simulate a rectifier. This rectifier converts the changing
voltage at the press switch into a constant voltage, which keeps the LED on.
The electricity supplied to your home by your electric company is actually a
changing voltage. Many electronic products use rectifier circuits to convert this
into a constant voltage like a battery provides.
You can replace the 1K Ω resistor with the 100Ω resistor (R1). This makes the
LED a little brighter but you have to press the switch faster to keep it on,
because the lower resistance drains the capacitor faster.